4,569 research outputs found
Probing the helium-graphite interaction
Two separate lines of investigation have recently converged to produce a highly detailed picture of the
behavior of helium atoms physisorbed on graphite basal plane surfaces. Atomic beam scattering experiments
on single crystals have yielded accurate values for the binding energies of several· states for both (^4)He and (^3)He, as well as matrix elements of the largest Fourier component of the periodic part of the interaction potential.
From these data, a complete three-dimensional description of the potential has been constructed, and the
energy band structure of a helium atom moving in this potential calculated. At the same time, accurate
thermodynamic measurements were made on submonolayer helium films adsorbed on Grafoil. The binding
energy and low-coverage specific heat deduced from these measurements are in excellent agreement with
those calculated from the band structures
Casimir Energies and Pressures for -function Potentials
The Casimir energies and pressures for a massless scalar field associated
with -function potentials in 1+1 and 3+1 dimensions are calculated. For
parallel plane surfaces, the results are finite, coincide with the pressures
associated with Dirichlet planes in the limit of strong coupling, and for weak
coupling do not possess a power-series expansion in 1+1 dimension. The relation
between Casimir energies and Casimir pressures is clarified,and the former are
shown to involve surface terms. The Casimir energy for a -function
spherical shell in 3+1 dimensions has an expression that reduces to the
familiar result for a Dirichlet shell in the strong-coupling limit. However,
the Casimir energy for finite coupling possesses a logarithmic divergence first
appearing in third order in the weak-coupling expansion, which seems
unremovable. The corresponding energies and pressures for a derivative of a
-function potential for the same spherical geometry generalizes the TM
contributions of electrodynamics. Cancellation of divergences can occur between
the TE (-function) and TM (derivative of -function) Casimir
energies. These results clarify recent discussions in the literature.Comment: 16 pages, 1 eps figure, uses REVTeX
Casimir energy, dispersion, and the Lifshitz formula
Despite suggestions to the contrary, we show in this paper that the usual
dispersive form of the electromagnetic energy must be used to derive the
Lifshitz force between parallel dielectric media. This conclusion follows from
the general form of the quantum vacuum energy, which is the basis of the
multiple-scattering formalism. As an illustration, we explicitly derive the
Lifshitz formula for the interaction between parallel dielectric semispaces,
including dispersion, starting from the expression for the total energy of the
system. The issues of constancy of the energy between parallel plates and of
the observability of electrostrictive forces are briefly addressed.Comment: 11 pages, no figure
Remark on the perturbative component of inclusive -decay
In the context of the inclusive -decay, we analyze various forms of
perturbative expansions which have appeared as modifications of the original
perturbative series. We argue that analytic perturbation theory, which combines
renormalization-group invariance and -analyticity, has significant merits
favoring its use to describe the perturbative component of -decay.Comment: 5 pages, ReVTEX, 2 eps figures. Revised paper includes clarifying
remarks and corrected references. To be published in Phys. Rev.
Redefining critical autism studies: A more inclusive interpretation
This article explores the definition of Critical Autism Studies and its inclusion in autistic scholarship. There has been critique of recent non-autistic literature for lacking autistic authorship, leading to doubts about its epistemological integrity due to misrepresentations of autistic culture and the neurodiversity movement. This article utilises the work of Arnold, Milton and O’Dell et al. to introduce an emancipatory definition to ensure the discipline is autistic led. In the process, we discuss the nature of autism studies and what constitutes critical literature. We propose Waltz’s interpretation of Critical Autism Studies as a working definition
Casimir effect for curved geometries: PFA validity limits
We compute Casimir interaction energies for the sphere-plate and
cylinder-plate configuration induced by scalar-field fluctuations with
Dirichlet boundary conditions. Based on a high-precision calculation using
worldline numerics, we quantitatively determine the validity bounds of the
proximity force approximation (PFA) on which the comparison between all
corresponding experiments and theory are based. We observe the quantitative
failure of the PFA on the 1% level for a curvature parameter a/R > 0.00755.
Even qualitatively, the PFA fails to predict reliably the correct sign of
genuine Casimir curvature effects. We conclude that data analysis of future
experiments aiming at a precision of 0.1% must no longer be based on the PFA.Comment: 4 pages, 4 figure
Mode-by-mode summation for the zero point electromagnetic energy of an infinite cylinder
Using the mode-by-mode summation technique the zero point energy of the
electromagnetic field is calculated for the boundary conditions given on the
surface of an infinite solid cylinder. It is assumed that the dielectric and
magnetic characteristics of the material which makes up the cylinder
and of that which makes up the surroundings obey the relation . With this
assumption all the divergences cancel. The divergences are regulated by making
use of zeta function techniques. Numerical calculations are carried out for a
dilute dielectric cylinder and for a perfectly conducting cylindrical shell.
The Casimir energy in the first case vanishes, and in the second is in complete
agreement with that obtained by DeRaad and Milton who employed a Green's
function technique with an ultraviolet regulator.Comment: REVTeX, 16 pages, no figures and tables; transcription error in
previous version corrected, giving a zero Casimir energy for a tenuous
cylinde
Attractive Casimir effect in an infrared modified gluon bag model
In this work, we are motivated by previous attempts to derive the vacuum
contribution to the bag energy in terms of familiar Casimir energy calculations
for spherical geometries. A simple infrared modified model is introduced which
allows studying the effects of the analytic structure as well as the geometry
in a clear manner. In this context, we show that if a class of infrared
vanishing effective gluon propagators is considered, then the renormalized
vacuum energy for a spherical bag is attractive, as required by the bag model
to adjust hadron spectroscopy.Comment: 7 pages. 1 figure. Accepted for publication in Physical Review D.
Revised version with improved analysis and presentation, references adde
The Adler Function for Light Quarks in Analytic Perturbation Theory
The method of analytic perturbation theory, which avoids the problem of
ghost-pole type singularities and gives a self-consistent description of both
spacelike and timelike regions, is applied to describe the "light" Adler
function corresponding to the non-strange vector channel of the inclusive decay
of the lepton. The role of threshold effects is investigated. The
behavior of the quark-antiquark system near threshold is described by using a
new relativistic resummation factor. It is shown that the method proposed leads
to good agreement with the ``experimental'' Adler function down to the lowest
energy scale.Comment: 13 pages, one ps figure, REVTe
Realizability of metamaterials with prescribed electric permittivity and magnetic permeability tensors
We show that any pair of real symmetric tensors \BGve and \BGm can be
realized as the effective electric permittivity and effective magnetic
permeability of a metamaterial at a given fixed frequency. The construction
starts with two extremely low loss metamaterials, with arbitrarily small
microstructure, whose existence is ensured by the work of Bouchitt{\'e} and
Bourel and Bouchitt\'e and Schweizer, one having at the given frequency a
permittivity tensor with exactly one negative eigenvalue, and a positive
permeability tensor, and the other having a positive permittivity tensor, and a
permeability tensor having exactly one negative eigenvalue. To achieve the
desired effective properties these materials are laminated together in a
hierarchical multiple rank laminate structure, with widely separated length
scales, and varying directions of lamination, but with the largest length scale
still much shorter than the wavelengths and attenuation lengths in the
macroscopic effective medium.Comment: 12 pages, no figure
- …